Definition of a planet
Sen—On the face of it, the question 'what is a planet?' would seem obvious and trivial. Planets orbit the Sun, or other stars if they're extrasolar planets—known as exoplanets. They're round in shape and have gravity. Regarding our own Solar System we long-thought we knew what a planet was, even when a new one was discovered. But in 2005 with the discovery of Eris, all that changed.
Since ancient times humanity knew of five planeta—from the Greek 'aster planetes', meaning 'wandering star'. They were Mercury, Venus, Mars, Jupiter and Saturn. The advent of telescopic astronomy in the 1600s meant astronomers could see these planets were not stars at all, but bodies in the Solar System. Uranus had been observed many times, but not known to be a planet until 1781. Then in 1846 Neptune was discovered by three astronomers in France, Germany and Britain. This brought the total number of planets in the Solar System (including Earth) to eight. In 1930, the discovery of Pluto by American astronomer Clyde Tombaugh increased this total by one. And since then schoolchildren learned that there were nine planets in the Solar System. This state of affairs seemed fixed and settled.
Though initially classed at the tenth planet, the discovery of Eris (official designation 136199 Eris) in early 2005 shattered this settled view and made astronomers realise they hadn't actually defined a planet.
There were two reasons why Eris's discovery caused such a problem. One is that like Pluto, Eris is a Kuiper Belt Object—a band of icy, rocky bodies beyond Neptune that number in their millions. The second is that with a diamater between 2,314 and 2,338 km, Eris is similar in size to Pluto. That means there could potentially be hundreds of objects as big as—or even larger than—Pluto. Should they all be considered planets?
Though not as large as Pluto, discoveries of other bodies such as 50000 Quaoar, 90377 Sedna, 136472 Makemake and 136108 Haumea meant that the International Astronomical Union (IAU)—the official organisation responsible for naming Solar System objects—had to come up with the first definition for 'planet'.
In August 2006 the 26th General Assembly of the IAU were close to announcing a 12-planet Solar System, with the addition of Eris, Pluto's moon Charon and the asteroid Ceres—the largest body in the Asteroid Belt—to the nine known planets.
But instead the IAU decided on eight official main-body planets in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune by applying its new definition of planet.
The IAU's definition of a planet was that it must meet four conditions, the Resolution stating: "The IAU therefore resolves that planets and other bodies in our Solar System, except satellites [one of the four conditions], be defined ... in the following way:
A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Applying this definition, there are eight planets in our Solar System. Pluto failed to meet the third condition. So if not a planet, what was it?
The IAU's answer was a new class of object they called dwarf-planet. A dwarf planet shared the first two criteria of a planet, but it did not need to meet the third condition of having cleared its orbital neighbourhood. A dwarf planet was also defined as not being a satellite.
Ceres, Eris, Makemake and Haumea were therefore classified as dwarf planets. And if Eris belonged to that category then by necessity, Pluto did too.
Anything else orbiting the Sun, that wasn't a moon, was known as a 'Small Solar System Body'. The definition for exoplanet is still a work in progress. Although controversial, these first definitions have stuck. But history shows that as our knowledge of the Solar System grows through discovery, these definitions may yet change.
Image that shows some dwarf planets. Some only show provisional designations as they weren't yet officially named (and some have yet to be so). There are also some asteroids and Kuiper Belt Objects along the bottom. Image credit: Internationa Astronomical Union/Martin Kommesser.